1. Field of the Invention
The present invention relates to a method of using a photocathode for emitting photoelectrons generated upon incidence of light, and a method of using an electron tube using the method of using a photocathode.
2. Related Background Art
In conventionally available electron tubes including photomultipliers, image intensifiers, and streak tubes, a photocathode consisting of an alkali metal compound or a Group III-V compound semiconductor is generally used.
Photoelectrons excited in such a photocathode upon incidence of light move while being diffused. The photoelectrons reach the electron emission surface via various routes without taking the shortest route. For this reason, the difference in moving distances between the photoelectrons directly results in differences (variations) in transit time of photoelectrons. After all, the differences in transit time of photoelectrons in the photocathode are caused by the limited thickness of the photocathode.
From the view point of quantum efficiency of photoelectric conversion, particularly when light having a relatively long wavelength is to be detected, light absorption in the photocathode occurs at a deep position from the light incident surface. Therefore, in a reflection type photocathode, as the wavelength of incident light becomes longer, the moving distance of photoelectrons reaching the electron emission surface becomes larger accordingly. In a transmission type photocathode, as the wavelength of incident light becomes longer, the photocathode must be made thicker.
In a photocathode, therefore, quantum efficiency in photoelectric conversion and differences in transit time of photoelectrons are contrary to each other. Photocathodes capable of improving both of them have not been put in practice yet.
There is a photocathode for detecting light having a relatively long wavelength, in which an InGaAsP active layer, an InP emitter layer, and an Ag protective layer are sequentially formed on an InP substrate. In this transition electron type photocathode, a bias voltage for optimizing the S/N ratio is applied on the basis of a balance between an increase in quantum efficiency of photoelectric conversion according to an increase in bias voltage, and an increase in dark current generated upon injection of holes from an electrode.
Note that a prior art associated with such a transition electron type photocathode is disclosed in, e.g., U.S. Pat. No. 3,958,143 in detail.